Utilization of thermoplastics in the manufacture of various molded articles is well known and widely exploited. Recent efforts to minimize part weight and reduce the amount of polymer required for a given piece have gained a great deal of attention. Generally, two approaches for reducing weight and material have evolved; first, use of a pressurized gas to “core-out” the center portion of the given piece; second, use of a nucleating agent and, or blowing agent, to induce foaming of the polymer. The hollow core and void areas of the foamed-plastic displace polymer material, thereby, creating a part equivalent in exterior dimension to a given solid plastic part using less polymer. An ancillary benefit of using foamed-plastic and hollow cores is that larger dimensioned molded articles can be manufactured with less sink marks and related surface deformations.
Foamed polymeric materials are well known, and typically are produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer and shaping the mixture with an extrusion die or closed mold. Exposure to atmospheric, or reduced pressure conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Under controlled conditions the cells can be made to remain isolated and a closed-cell foamed material results. Under other, typically more violent foaming conditions, usually associated with less complex and less expensive molding processes, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes chemical decomposition in the polymer material causing formation of a gas.
Use of uniform, microcellular, foamed-plastic results in molded parts with more visually pleasing surface finish than does an open cell foamed-plastic. However, non-conventional, more expensive, complex molding equipment and related processes are required to produce uniform, microcellular, foamed-plastics.
U.S. Pat. No. 6,169,122 B1, to Blizard et al., the disclosure of which is incorporated herein by reference thereto, discloses a method of producing articles of foamed-plastic wherein the foamed-plastic is formulated in such a way as to induce a uniform density of microcellular voids. The Blizard et al. disclosure discusses in length the various approaches taken in an effort to produce a foamed-plastic article with acceptable surface characteristics. Blizard et al. combines a chemical blowing agent and physical blowing agent to produce a foamed-plastic which exhibits uniform, microcellular, characteristics. The associated molding apparatus is non-conventional, expensive and complex.
The process of hollowing out the core may be used to provide the above benefits. However it may lead to the production of undesirable visual surface characteristics. U.S. Pat. No. 5,639,405, to Erikson, the disclosure of which is incorporated herein by reference thereto, provides an enlightened discussion of various apparatus for molding a hollow core plastic article. As disclosed in Erikson, a great deal of complexity is added to the related molding equipment in an effort to time the injection of molten plastic with the injection of gas. In the method of Erikson, supra, the timing of injecting gas with respect to injection of molten plastic is critical with regard to minimizing the resulting undesirable surface effects and maximizing resulting part integrity. The molding equipment used in Erikson employs a host of sensors and a corresponding control system to automatically adjust the injection timing.
Many of the molded plastic parts resulting from known manufacturing processes which incorporate coring and foaming steps have not yet achieved show-face characteristics with corresponding production rates acceptable for producing automotive products. Oftentimes, known coring operations with acceptable production rates induce swirling, inconsistent, patterns on the exterior surfaces of the corresponding parts and are sometimes susceptible to leaving cellular void areas on the corresponding part surfaces.
Therefore, there remains a need in the art for an economically viable method of manufacturing a light weight molded plastic part with a class A show-face surface while obtaining an acceptably high production rate. A further desire is to retain use of conventional molding equipment without added complex, expensive, systems.